We have measured static lung volumes after heart-lung transplantation (HLT) in seven patients with cystic fibrosis (CF) (Group 1), three patients with chronic hyperinflation due to diseases other than CF (Group 2), and six patients with primary pulmonary hypertension (PPH) (Group 3). Total lung capacity was within normal limits at 1 yr after surgery in all patients. Similarly, FRC was within the normal range in Groups 2 and 3. On the other hand, patients with CF showed a persistent increase in FRC; at 1 yr after HLT, FRC averaged 4.13 +/- 0.52 L compared with a predicted value of 3.20 +/- 0.23 L (p < 0.01). The postoperative static pressure-volume curve of the lung in the patients with CF was superimposed on the predicted one, suggesting that the increased FRC originated in the chest wall. Additional studies with computerized tomographic scans demonstrated that the rib cage anteroposterior diameter at FRC averaged 12.1 +/- 1.6 cm in patients with CF, 9.5 +/- 1.2 cm in patients with PPH, and 9.4 +/- 0.7 cm in a group of healthy subjects matched with the patients with CF (p < 0.01). We conclude that after HLT, patients with CF show persistent hyperinflation due to rib cage expansion along the anteroposterior dimension. This shape change may represent a structural adaptation that occurs in response to chronic pulmonary hyperinflation acquired during rib cage growth.